When drilling a well, various logging tools and sensors are used to investigate the fluids contained within or seeping from borehole walls in order to determine if a formation contains oil or gas. One such tool is a nuclear magnetic resonance (NMR) logging tool. NMR logging tools can be used to determine properties of earth formation, such as the fractional volume of mobile fluid within the pore space, hydrogen density, diffusion, and the fractional volume of pore space (porosity).
NMR tools measure the relaxation rates of hydrogen atoms in the pore spaces of earth formations by emitting a sequence of radio-frequency pulses into a formation subject to an applied magnetic field and then monitoring the response. The amplitude of the response is measured by the NMR tool and is proportional to the mean density of hydrogen nuclei in the fluid that occupies the pore spaces in the probed volume. Because the hydrogen densities in water and in liquid hydrocarbons are known, the detected NMR signal is proportional to the volume fraction of liquid occupying the pore space.
Most NMR logging tools are tuned to detect hydrogen resonance signals (from either water or hydrocarbons), because hydrogen nuclei are the most abundant and easily detectable. Depending on factors such as the surrounding chemical environment and molecular size of the parent molecule, hydrogen nuclei exhibit different dynamic properties (e.g., diffusion rate and can tumbling/rotation rate). The different dynamic properties of these nuclei manifest themselves in different nuclear spin relaxation times that may be categorized as spin-lattice relaxation time (T1), spin-spin relaxation time (T2), and overall diffusion of the nuclei in the surrounding media (D). For example, molecules in viscous oils cannot diffuse or tumble as fast as those in light oils. As a result, hydrogen nuclei present in diffusion restricted environments (i.e., condensed or liquid phases) have shorter relative relaxation times than hydrogen nuclei in chemical environments having higher average diffusion such as gases. In a particular example, the data acquired from the NMR logging tool may provide valuable information about the molecular properties of hydrocarbons and aqueous fluids within earthen formations.